Guides / Materials
Why your aluminium fittings corrode where they touch carbon fibre
September 17, 2026
A carbon tube clamped in an aluminium fitting is the most common joint on a modern airframe. It is also a battery.
Carbon fibre conducts electricity and sits near the noble end of the galvanic series. Aluminium sits near the active end. Put them in contact with an electrolyte, which means rain, condensation, agricultural spray or salt air, and you have a cell with roughly a volt across it. The aluminium is the anode, so the aluminium is what dissolves. The finish on the metal side of this joint is covered in our guide to drone parts finishing. For the choice between carbon fibre and aluminium in the first place, see carbon fibre vs aluminium for drone frames.
What it looks like on a real aircraft
It does not look like rust. It looks like:
- White powder around the clamp faces and under the bolt heads.
- A clamp that will not come off the tube because the corrosion product has grown in the gap and locked it.
- Pitting on the bore of a boom clamp, always worst where water sat.
- A bolted joint that loses preload over a season because the material under the washer has been eaten.
The damage concentrates where the area ratio is worst. A small aluminium part against a large carbon surface corrodes faster than a large aluminium part against a small carbon one, because the whole cathode area drives current into a small anode.
Five fixes, in the order they usually make sense
1. Keep the electrolyte out. No water, no cell. On a sealed airframe this can be enough. On an agricultural aircraft it never is, so do not rely on it alone.
2. Insulate the joint. A thin glass fibre layer, a polymer shim, an adhesive film or a machined POM sleeve breaks the electrical path. This is the classic aerospace fix, a ply of glass between the carbon and the metal. On a machined clamp it means designing in space for the sleeve, roughly 0.3 to 0.5 mm, and holding the bore to suit the sleeve rather than the tube.
3. Anodise, then seal. Hard anodise gives a thicker oxide than type 2 and it stays put. It slows the reaction, it does not stop it, and it stops working where the layer is broken. Anodise plus a wet sealant or a primer under the clamp faces is the normal combination. The growth that hard anodise adds to a bore is covered in anodising UAV parts.
4. Use titanium or stainless for the fitting. Titanium is the clean answer for small fittings and fasteners. Stainless sits closer to carbon than aluminium does, so it is better than aluminium and worse than titanium, and it is heavy. See titanium Grade 5 on a UAV.
5. Add a sacrificial part. A zinc or aluminium washer that is cheap to replace corrodes before the structure does. This works on maintained fleets and is pointless on an aircraft nobody opens.
Fasteners are the part people forget
A stainless bolt through a carbon plate into an aluminium boss is three metals and two joints. In practice:
- Use a washer that insulates, or at least spreads the contact.
- Wet assemble with a sealant where the aircraft sees weather. It fills the gap that would otherwise hold water.
- Avoid stainless bolts in bare aluminium bosses on a weather exposed joint. Either isolate them or use a compatible fastener.
- Thread locking compound is not a corrosion barrier, although it helps by filling the thread.
What this means for the machined part
Most of the fixes above have to exist on the drawing before the part is cut.
| Decision | What it changes on the part |
|---|---|
| Insulating sleeve | Bore grows by two wall thicknesses, and the tolerance moves to the sleeve fit |
| Hard anodise | Bore grows by roughly half the coating thickness, so machine undersize or mask |
| Sealant under the clamp | Needs a face that is flat and not polished, and often a small relief for squeeze out |
| Sacrificial washer | A counterbore or a flat landing so the washer sits properly |
| Titanium fitting | Different material, different cost and different machining, quoted separately |
The mistake we see most often on an RFQ is a bore dimensioned for the carbon tube, hard anodise called out, and no mention of the sleeve. Three of those decisions fight each other and the part arrives either tight or loose.
A quick specification checklist
- State the mating material. If a machined part clamps carbon, say so on the drawing.
- Say whether the bore is finished before or after anodising.
- If there is a sleeve or an insulating layer, give its thickness and put the fit on the sleeve.
- Call out the faces that get sealant, so they are not polished or anodised where they should not be.
- For aircraft that fly over sea or spray crops, say so. It changes the finish recommendation and it is cheaper to hear before the lot runs.
Send the model with a note about what the part touches and we will raise anything that looks like a corrosion trap before we price it. The boom clamp article covers the fit side of the same joint.
Frequently asked questions
How fast does aluminium corrode against carbon fibre?
It depends entirely on the electrolyte. Dry indoor use, almost never. Rain and humidity, over months. Agricultural spray or coastal salt air, weeks to months, with published work showing aluminium corroding many times faster in contact with carbon than on its own.
Is anodising enough to stop it?
Not on its own. Anodise is an oxide layer, so it slows the reaction, but any scratch, any machined face left bare after anodising and any fastener hole breaks the barrier. Anodise plus a sealant or an insulating layer is the working answer.
Does titanium have the same problem?
Much less. Titanium sits close to carbon on the galvanic series, so the driving voltage is small. That is one of the main reasons titanium fittings show up in composite airframes.
Need this part quoted?
Send the model and the quantity. Price and lead time come back within 24 business hours.